There may indeed be a legitimate hardware reason to keep Siri limited to the iPhone 4S, suggests iFixit. On reexamining the phone, the repair firm says it paid more attention to an infrared LED that turned out to be a proximity sensor. Located next to an ambient light unit, the 4S proximity sensor judges how close a person's face is. The 4S implementation is unique from other iPhones, according to iFixit, in that it is always on as long as the screen is on.
